To add to Dom's answer, I believe that the progression was miswritten. Bb really should have been placed there (at least if these three chords are isolated in their own progression and not surrounded by other distant chords). Bb major fits perfectly in the key of D minor; A#, while having the same pitch, really doesn't fit, unless it was going to do something to modulate away, like leading to D# (but that opens up a whole new can of worms ;) ).
BTW, imagine this as the song "Stray Cat Strut." These chords would make a nice progression and would clarify to you why Bb is the better-named chord (as opposed to A#):
i VII VI V7
(which if put in D minor would be)
Dm C Bb A7
Dm C A# A7 ...
would look strange, wouldn't it, as you're skipping any B in between C and A!